While on a hike with my kids, we discovered a light, springy green plant growing in small clumps on the trail. According to a signpost, the plant is commonly called deer moss, but it’s not actually a moss at all. It’s a lichen. A lichen is a fungus and an alga growing together in a mutualistic relationship in which both organisms benefit from each other. Neither the fungus nor the alga can survive on its own, but together they form a hardy plant that can live in some alpine areas for up to 4,500 years. Because the plant can withstand drought and low temperatures, it’s one of the only food sources for caribou (reindeer) in deep winter.